Conventional lithography systems include, among other things, an illumination system to produce a uniform intensity distribution of a received laser beam. It is desirable that the resulting illumination be as uniform as possible and that any uniformity errors be kept as small as possible. Illumination uniformity influences the ability of an illumination system to produce uniform line widths across an entire exposure field. Illumination uniformity errors can significantly impact the quality of devices produced by the lithography system.
Techniques for correcting uniformity include correction systems that have multiple correction elements such as plates inserted from opposites of an illumination slot. These correction elements have non-zero attenuation (e.g., 90%). However, due to various constraints, a gap exists between adjacent correction elements. The gaps between adjacent correction elements generate unwanted optical effects such as gap ripples and shadows. Because each gap has a 0% attenuation (or 100% transmission) and the correction elements have non-zero attenuation, light through the gaps generate streaks or bands of greater intensity on the substrate. The bands of greater intensity impact the width of lines in the exposure field. Furthermore, each correction element has a finite thickness. Thus, each correction elements has a plurality of edges. If light is coming in on an angle (i.e., larger sigma), part of the light reflects off the edge, casting a shadow on the substrate.
Therefore, what is needed is a uniformity correction system that compensates for optical effects created by gaps between adjacent correction elements, that provides increased uniformity across the slot, and that improves critical dimensions.